Basically, I want to inform the user whether the script succeeded or not (as it has a certain chance to fail). To do so, I added a filter object behind the Run Script and post a notification.
I read a lot about the structure of workflows this evening and I think that the main issue is/was that I was calling the complete script instead of putting the steps of the script into the Run Script element.
My new approach is now:
1) I added a variable to the workflow
2) I put the various steps of my bash script into the run script element. I can check for errors and set the workflow variable accordingly
3) In the filter I can then check for the value of the workflow variable and post a message.
So far, this is not yet runnning. But I' hoping to be on a good way...